X-ray images are decreased in contrast by X-rays scattered from objects being imaged; this is referred to as “forward scatter”. In an effort to overcome forward scatter, anti-scatter grids have long been used (Gustov Bucky, U.S. Pat. No. 1,164,987) to absorb the scattered X-rays while passing the primary X-rays, which produce the desired image. A problem with using grids, however, is that whenever the X-ray radiation detection panel resolution is comparable to or higher than the spacing of the grid, an image artifact from the grid may be seen. Bucky recognized this problem, which he addressed by moving the anti-scatter grid during exposure to eliminate grid image artifacts by blurring the image of the anti-scatter grid (but not of the object). Subsequent improvements to the construction of anti-scatter grids have reduced the need to move the grid, thereby simplifying the apparatus and timing between the anti-scatter grid motion and X-ray source. For all of these early systems, images were recorded on radiographic films using traditional silver halide technology.
More recently, digital radiographic imaging using radiation detection panels comprising a two-dimensional array of tiny sensors to capture a radiation-generated image have come into common use. The radiation is imagewise modulated as it passes through an object having varying radiation absorption areas. Information representing an image is typically captured as a charge distribution stored in a plurality of charge storage capacitors in individual sensors, arrayed in a two dimensional matrix, hereinafter referred to as a digital sensor array.
However, Moiré pattern artifacts can be introduced when image capture is accomplished through this means if an anti-scatter grid is used, or when film images are digitized. (The Essential Physics of Medical Imaging, Jerrold T Bushberg, J. Anthony Seibert, Edwin M. Leidholdt, Jr., and John M. Boone. ©1994 Williams & Wilkins, Baltimore, pg. 162 ff.). Thus when the X-ray radiation detection panel employs a digital sensor array, thereby generating a two dimensional array of picture elements, the beat between the spatial frequency of the sensors and that of the anti-scatter grid gives rise to an interference pattern having a low spatial frequency, i.e. a Moiré pattern.
Several approaches have been taken to attempt to solve this problem, all of them involving improvements in the design and/or motion of the grids. One described in U.S. Pat. No. 5,666,395 to Tsukamoto et al. teaches Moiré pattern prevention with a static linear grid having a grid pitch that is an integer fraction of the sensor pitch.
As noted above, the approach originally proposed by Bucky in U.S. Pat. No. 1,164,987, describes moving the anti-scatter grid during radiation exposure to blur the artifact images generated by the grid. This approach is limited by the fact that, in modern radiographic equipment, the exposure time is determined by automated exposure control devices. The total exposure time is therefore unknown, making it difficult to time the grid motion to cover the entire exposure period.
A third approach, designed to overcome the problem of not knowing the total exposure time required for a given imaging event, is described by Lee et al in U.S. Pat. No. 6,181,773, and involves the use of a grid driven with a variable speed profile.
Despite these advances however, X-ray imaging that requires high radiation doses due to thick samples or a need for better contrast detail suffers image quality degradation due to scatter radiation. Sometimes the scatter radiation is in fact greater than the contrast detail inherent in the image. Moreover, in the case of low contrast detail imaging where signal strength is increased to improve image quality, the use of an anti-scatter grid is usually not possible. In such systems, the combination of short wavelength X-ray and exposure times of several seconds make it difficult to use either a single-stroke or a multi-stroke grid.
In addition to the forward scatter generated by the object being imaged, “back scatter” resulting from X-rays impinging on components behind the image sensor sheet causes further image degradation. In U.S. Pat. No. 5,804,832, Crowell teaches the use of thin strips of lead to protect electronic components from damage by impinging radiation, but such strips are outside the image area (if in front of the sensor sheet) or cover only part of the image area (if behind the sensor sheet), such that only a portion of the back scatter is blocked. Thus there continues to be a need for practical means of reducing both forward and back scatter, particularly in low contrast detail imaging applications.